Party system institutionalization has been traditionally viewed as an essential condition for the consolidation of democracy (Mainwaring, 1999; Hicken and Kuchonta, 2014; Casal Bértoa, 2017). However, there is a far less agreement in the literature about what institutionalizes party systems in the first place. Seeking to fill this gap, the current paper, building on a mixed-method approach, aims to provide an answer to the question of how such institutionalization occurs and why the degrees of institutionalization vary across countries. Specifically, we conduct a comparative analysis of the process of party system closure in 21 Asian democracies, relatively overlooked regional cases in this literature, since the end of WWII until 2021. Asia provides excellent cases for such analysis, because its democracies show diverse degrees of institutionalization in electoral and governmental dimensions of party systems (Lee and Casal Bértoa, 2021). In the end, this variety will lead us to observe wide inter-state variation in party system institutionalization. Using a recent innovative index of measuring party system institutionalization by Casal Bertoa and Enyedi (2021), the paper will first present differences in the levels of party system institutionalization in Asia in a comparative perspective. Then, after a thorough review of the literature on different sources of systemic institutionalization (Mainwaring and Bizarro, 2018; Chiaramonte and Emanuele, 2019; Enyedi and Casal Bértoa, 2020), it will state 11 different “working” hypotheses (i.e. fragmentation, polarization, electoral system design, party funding, party institutionalization, constitutional type of regime, social cleavages, economic development, historical legacies, democratic experience and time of transition). In our empirical analysis, we test these hypotheses using statistical analysis, with a particular focus on the role different historical (i.e. authoritarian, colonial) legacies have exerted over the process of party system closure in Asia.